Common hydrocarbon fuel burning appliances use natural gas (NG) or liquid petroleum gas (LPG) or heavy oil (HO) for heating and normally use electrical power for control and indication. When faulty, or inadequately ventilated, operation of such appliances can result in the emission of carbon monoxide (CO) gas. As CO is colourless, odourless and tasteless, it is advisable to mount domestic CO detectors at ceiling level, away from dead spaces or obstructions for the effective detection of less dense CO gas. However a compromise is normally made between the CO detectors being sited near potential sources of CO gas and the alarm being loud enough to be heard throughout the building. Such detectors are mainly battery powered, enabling a simple low cost installation and usually do not require battery replacement during the detectors operation life.
When an integrated response level is exceeded, the CO detector is operable to output an audio alarm signal, which at best would result in the manual isolation of power to an electrically controlled appliance, which is likely to expose the responsible person to a greater level of CO gas. This can result in dizziness, confusion, unconsciousness, brain damage and ultimately death.
If no manual isolation of power to the appliance is achieved, the alarm will continue to sound until the battery is exhausted or the detector reaches the end of a predetermined alarm cycle. During this time, the appliance may continue to operate and raise the concentration of CO levels. This may occur if the building is unoccupied, if the alarm is insufficiently audible or if the occupants are heavy sleepers, intoxicated, have a hearing impairment, infirm or infant. It is therefore desirable to provide an automated isolation device operable to the safe disconnection of power to an electrically controlled appliance in response to the activation of an alarm produced by a carbon monoxide (CO) detector.
In this context, WO 2010/136808 discloses a device for detecting and responding to an audio alarm from a smoke detector. The device detects audio signals, processes the audio signals and thus identifies audio alarm signals. In order to avoid false alarms, there is a need to discriminate between alarm signals and other noise signals, for example from a TV, radio or other unrelated alarms. In this case, discrimination is made by filtering the detected signal, the pass band of the filter centred on the alarm frequency. However, a compromise is made in the frequency discrimination to accommodate the wide variations in the alarm signal frequency. This is mainly due to the initial accuracy of the piezoelectric sounder used in the detector and the associated frequency drift with temperature and age. Typically the input filter needs to have a pass-band of at least 3.2 KHz+/−500 Hz or a bandwidth of a minimum of 1 KHz, giving a very low Q of about 3. Given that significant levels of audio noise could exist near to the pass-band, or even in the pass-band, a large processing delay may be required to reject a false alarm, which can make a test of the system impractical. It is also possible that a real alarm signal may actually be at a similar or even lower level than a noise source, causing the alarm signal to be masked by the noise and to remain undetected. In view of the above the device of WO 2010/136808 is unreliable in practice.
An alternative device is disclosed in WO2011/014694 providing an improvement to the detection signal to noise ratio, by using many selective filters, to cover the same pass-band. Assuming each filter had an individual pass-band of +/−25 Hz, then an impractical 20 high Q filters would be required to cover the expected frequency range. Furthermore the device will need to undertake complex analysis to monitor all the outputs of each filter. As such, this arrangement is relatively complex, expensive and impractical.
It is therefore an object of the present invention to provide an isolation device that at least partially overcomes or alleviates some of the above problems.